Evidence of quadrupole-quadrupole interactions in ultracold gases

Van der Waals interactions are interactions between dipoles. Similarly, quadrupole-quadrupole interactions are interactions between quadrupoles. In this article, we focus on the interactions between two dipoles or two quadrupoles. Classically, we treat one Rydberg atom as a dipole; an outer excited electron and an ion core are the two poles of a dipole. Quantum mechanically, we consider Rydberg transition dipoles. Therefore, dipole-dipole interactions are the interactions between two Rydberg atoms. Rydberg atoms have quadrupole components; consequently, the interactions between two Rydberg atoms have quadrupole-quadrupole interaction components. In this article, we examine the dipole-dipole and quadrupole-quadrupole contribution to the interactions between ultracold Rydberg atoms. It is shown that the evidence of quadrupole-blockade has been observed, which is essential for fabricating more compact quantum computers, quantum electronics, as well as quantum sensing.